Inorganic salt-resin conductive coatings for electrophotographic paper

ABSTRACT

Electrophotographic paper, having thereon conductive coatings applied from an aqueous coating composition containing an Nvinylpyrrolidone polymer, polyvinyl acetate emulsion and an inorganic salt-polyhydric alcohol system.

States Patent INORGANIC SALT-RESIN CONDUCTIVE COATINGS lFOR ELECTROPHOTOGRAPHIC PAPER 5 Claims, No Drawings lU.S. Ci 96/l.5, 117/201,117/218,117/219,1l7/161UIN, 162/138, 260/17.4

lint. C1 G03g 5/00, 003g 7/00 Field of Search 96/1.5;

[56] References Cited UNITED STATES PATENTS 2,901,348 8/1959 Dessaner et a1 96/l.5 X 3,212,931 10/1965 Kubata et al 117/218 3,295,967 1/1967 Schoenfeld. 96/1.5 3,331,687 7/1967 Kosche.... 96/1.5 3,403,116 /1968 Ream 96/1.5 X 3,489,597 l/1970 Paker 117/161 UIN FOREIGN PATENTS 789,802 l/1958 Great Britain 96/1.5 1,062,092 3/1967 Great Britain.. 6/l.5

234,016 8/1959 Australia 96/1.5

Primary Examiner-George F. Lesme-s Assistant Examiner-John R. Miller An0rneys-George L. Tone, Walter C. Kehm and Samson B.

Leavitt ABSTRACT: Electrophotographic paper, having thereon conductive coatings applied from an aqueous coating composition containing an N-vinylpyrrolidone polymer, polyvinyl acetate emulsion and an inorganic salt-polyhydric alcohol system.

INORGANIC SALT-RESIN CONDUC'MVE COATINGS FOR ELECTROIPHOTOGRAPHIC PAPER This invention relates to novel coatings for solvent-based electrophotographic paper and, in particular, to both the coating solutions therefor and the resulting coated electrophotographic paper containing an inorganic salt-polyhydric alcohol ingredient.

Electrophotographic recording is a process for producing visible images involving the conversion by the imagewise exposure to light of a uniformly electrostatically charged surface intoa surface having a patterned configuration of electrostatically charged and discharged areas. Subsequent development of this patterned surface results in reproduction of the original image.

More particularly and as disclosed, for example, in US. Pat. No. 3,052,539, the electrophotographic recording material is charged in the dark by exposing its photoconductive zinc oxide pigment-coated surface to a charging device such as an arrangement of fine wires connected to a high-voltage DC source while the paper base is supported on a grounded metal plate. The pigment-coated surface, bearing a uniform electrostatic charge, is then exposed imagewise to light (e.g. by projection) whereby the light-exposed areas are rendered conductive, and their electrostatic charge is withdrawn through the conductive base and the grounded support, to yield a residual electrostatic charge pattern constituting a latent image of the projected subject matter. The image is then developed by application of a colored powder with an opposite charge, which adheres to the charged areas of the latent image. The powder may be dusted on the image-bearing surface or applied in suspension from nonconductive liquid, the excess powder being removed to yield a positive image of the projected subject matter. If a powder is used having the same charge as electrostatic image, it is repelled thereby and adheres instead to the remaining areas to provide a negative image of the projected subject matter. The powder is preferably fusible so that it may be caused to adhere permanently to the recording sheet by temporarily heating (e.g., with infrared radiation).

Coating compositions for the preparation of electrophotographic recording materials may be applied from either organic solvent solutions or from aqueous emulsions. There are, of course, many advantages to the use of aqueous emulsions among which are reduced costs, ease of application and the improved safety factor from the standpoint of both nonflammability and reduced toxicity.

it is in the area of development that complications have arisen, especially where the sheet material base is paper or other semiporous substrate. There are several processes for development as discussed above, but the mode of application of primary concern in the practice of this invention is that using liquid dyes or colored materials dispersed in a liquid vehicle. Since these liquid toner particles must be substantially immiscible with the vehicle or carrier liquid but capable of being dispersed therein in the form of very small droplets, the concentration ratio of vehicle to toner is critical. In addition the vehicle should be a neutral organic liquid, highly insulating in nature-that is having high electrical resistivity and low dielectric constant. This high resistivity is necessary to avoid the discharge of the latent electrostatic image.

One drawback to the use of these liquid toners is the absorption of the toner or its vehicle into the porous base which disturbs the liquid toner-vehicle balance and results in loss of contrast and sharpness in the developed, image. This is true to a degree in the case of any paper support and especially true where the sensitizing solution and other coating solutions are applied from aqueous emulsions.

These drawbacks have been overcome in the case of aqueous-based paper by the use of certain vinylpyrrolidonestyrene/butadiene polymers for the application of nonconductive barrier layers and, alternatively for the application of conductive layers, by the addition of quaternary ammonium salts thereto. In the case of solvent-based paper, the drawbacks were overcome by the quaternization of copolymcrs of vinylpyridine/vinylpyrrolidone with certain quaternizing agents.

However, even with the significant improvements afforded by the carrier and conductive coatings described above, there still remained several problems with solvent-based papers attributable chiefly to the specialized uses to which the papers are put. In general the conductive-coated solvent-based papers have undesirable glossy backs and excessive sensitivity to humidity. In copying machines utilizing liquid toner solutions such as the Bruning Copytron 2000, on the one hand, problems are odor, back curl and a 3-inch problem while in copying machines utilizing dry toner such as the Apeco Dial- A-Copy, on the other hand, odor, back-blistering and roll feed trouble have been the problems. l-leretofore these problems were minimized by preparing specially coated papers for each type of copying machine.

It has been found, however, that by the process of this invention such problems can not only be overcome thereby providing one paper suitable for use in both wet and dry process machines, but providing also a resulting conductive film-coated paper having properties superior to any solventbased paper heretofore available on a commercial basis.

Therefore this invention is based on the discovery that the replacement of the conductive resin in the aforesaid conductive-treating solutions by an inorganic salt-polyhydric alcohol system results in coated papers having greatly improved properties. Among these improvements are:

a. excellent solvent and dispersant hold-out,

b. good conductivity,

c. acceptable moisture retention and d. minimal sensitivity to humidity changes over a relative humidity range of 16-80percent.

These results are all the more surprising since previous evaluations using inorganic salts alone had not only been unsuccessful but were, in fact, shown to have a deleterious effect on the coated paper due to the resulting sensitivity to humidity. Thus, totally unexpected results arise from the unique combination of a polyhydric alcohol having moisture-regulating properties with an inorganic salt.

A typical conductive barrier film-forming solution is composed of the following ingredients in the range and preferred quantities as listed:

While this invention is not limited to any specific inorganic salts, it is delimited to those containing monovalent cations because of the known hygroscopic or deliquescent properties of inorganic salts containing polyvalent cations and, in particular, of those containing divalent cations. To eliminate or at least minimize these aforesaid deleterious effects, a high purity criterion for the inorganic salt was established. One example of such compound is sodium chloride of 99.95 percent purity sold by Morton Salt Co. under the trade name Culinox 999. Other acceptable salts include compounds containing the ammonium ion or cations of the elements of Group I of the Periodic Table, i.e. lithium, sodium, potassium, rubidium and caesium with such anionic moieties as chloride, bromide, nitrate, sulfate, phosphate and other nonvolatile, conductive lOIlS.

Suitable polyhydric alcohols include glycerol, glycols such as ethylene or propylene glycol, 1,4-butanediol and polyethylene glycols, sold, for example, under the trademarks Gafanol E 200 and Gafanol E 300.

Since the paper surface in liquid toner systems must be completely immersed in the developer solutions, a barrier layer coating must be applied thereon. [t has been found that an essential constituent in the coating compositions is a polyvinylpyrrolidone polymer. Examples of some vinylpyrrolidone polymers which are acceptable include:

a. Poly-N-vinylpyrrolidone,

length of this free travel path is directly relatable to the efficiency of the various coatings. Measurements are made on both the wire and felt sides of the paper.

b. N-vinylpyrrolidone-vinyl acetate copolymer 10:90), 5 Resistivity Test Nvinylpynolidone'vinyl acetate Copolymer (25:75 Data were also obtained on both surface and bulk resistivid. N-vinylpyrrolidone-vinyl acetate copolymer (30:70), N"'1'{Ylpyrrolidone'ethylacrylme copolymer (30:70) and Surface resistivity was calculated from measurements made I?"Wnylpyfrohdone'ethyiacrylate (30:70) 10 of the voltage developed between a pair of l-inch brass elecm'xed wlth poly'NvmylpyrmhdoneVmyl acetate trodes spaced 1 inch apart when placed on a paper sample 2 I mounted on an insulated surface. Bulk resistivity, i.e. the ref gwen m the parentheses mdcate the propomons by sistivity through the thickness of the paper, was calculated in welghtfifthe f f monowers' an analogous manner with the paper placed on a conductive Addmorfal mgredems polyvmyl came, acetylated Starch surface. In this procedure, however, the recorded voltage was and polyvlxfyl alcohol add body gloss propemes to that developed between the same pair of electrodes so posipaper substitute or import the required viscosity to the coating tiohed that one was on the paper and the Other was on the com composmons' ductive surface. These voltage measurements which were, in sevtfral exmples are Set forth below luustrat the nature turn, converted into resistivity, were determined over a range Ofthe mvemlon and the method ofcarrymg of humidities. Since they are indicative of the static susceptibility of the paper surface, they are, of course, indicative of L prepmation fc i Solutions moisture content and, hence, humidity sensitivity.

The coating compositions listed below were used in the ex- Examples 14 amples that follow. These compositions were formulated by These examples illustrate the beneficial effects to be ob- Standard Chemical Procedures, the Salt was dissolved in tained in using coating compositions containing various inorwater and added to a starch-polyvinyl alcohol slurry. This mixganic lt i bination with a polyhydric alcohol-4n this ture was then heated to and maintained at 195 F. for 15 ase, glycerol. minutes. The resulting solution was cooled to 120 F. and the Coating compositions prepared as described above were apother ingredients were added followed by dilution of the mixplied to various paper surfaces using the air knife techniques, ture to the desired percent solids (15-35 percent). The formuat such a rate as to yield a coating, when dry, of4.5 pounds for lations are identified by letter for purposes of convenience of 3,000 square feet. reference to indicate these compositions in the examples Solvent and toner holdout were determined by the below. The higher quantities for the ingredients in formulation procedure described above. The coatings offered excellent re- E were necessary to achieve the required percent solids solusistance to both solvent and toner absorption or penetration. tion for size press purposes. Surface and bulk resistivities, determined at several humidities Formulation. A B C D E J K Mole ratio salt/glycol. 2. 24 1.05 1.15 l. 18 NaCl... 3 25 2.28 4.7 3 3 Polyviny if. H 0.5 P0ly(viny pyrrolidone/vinyl acetat 3.3 2.6 6.9 4.3 4.3 Po1y(vinyl acetate). 5. 5 3. 3. 48 7. 2 4. 4 4. 4 Acetylated starch... 3 2. 25 2.28 4.7 3 3 Poly(vlny1 alcohol)... 3 3. 75 3.74 7. 9 4. 8 4. 8 Glycerol.... 2.1 3.36 3.1 6.3 1,4-butanediol Ethyl glycol GatanolE200 3.!) Ga1anolE300.. H 3. 11:0 to make 100 100 100 100 100 100 10 [L S l d Toner fl ld T using the procedure described earlier, indicated that the coated papers were acceptable for electrophotographic use in An empirical test was developed to measure the efficiency either wet (liquid) or dry development processes of the various coatings in resisting both solvent and liquid dye Details as to the compositions of the coatings, the Surface Penetrauons' coated and the test data obtained are tabulated below.

Resistivity (ohmsXlO'lsq. inch) Holdout (111111.) 20% RH 50% RH 80% RH Solvent Toner Surface Surface Surface Formu- Paper a Example lation Salt surface Felt Wire Felt Wire Felt Wire Bulk Felt Wiic Bulk Felt Wirc Bulk 1.. A NaCl DiflZO. 66 71 0.1 0. l 0.05 2... B NaCl Pattern... 62 57 60 58 0.13 0.15 0.047 4. F NH4C1 D1320... 7 73 78 72 11 8 1. 7 0. l7 0. ll 0. 058 0012 0 l 3. G NaNOa do 82 79 73 74 20 2 2. 7 0. 36 0. 37 0. l0 0010 0018 00089 A frame assembly, having an 8%Xl l-inch sheet of coated paper tightly clamped thereon, is positioned on a support with the surface of the paper at a 45-degree angle to the horizon.

A funnel connected through a valve to a hypodermic syringe whose tip delivers a 2 mm. diameter drop is positioned above the paper with the tip 12 mm. from the papers surface and perpendicular to the horizon.

in operation, a drop from the pipette strikes the paper sur- Examples 5-9 These examples show that various polyhydric alcohols can be used in the coating formulations.

Coatings were applied using the air knife procedure.

Solvent and toner holdout and resistivities were determined by the procedures described above. The coating offered excellent resistance to both solvent and toner absorption. Surface face and travels until completely absorbed or dried. The 75 and bulk resistivities, determined at several humidities, in-

dicated that the coated papers were acceptable for electrophotographic use in either wet or dry developed processes.

Details as to the compositions of the coatings, the surfaces coated and the test data obtained are tabulated below. No holdout data is available for examples 7 and 8. However, the retive layer applied from an aqueous coating composition consisting essentially of 2 to 4 percent by weight of an inorganic salt, 2 to 4 percent by weight ofa polyhydric alcohol selected from the group consisting of glycerol, ethylene glycol,

5 propylene glycol, 1,4-butanediol and polyethylene glycol, sistivities indicate that such coatings would be acceptable for 0.25 to 5 percent by weight of an N-vinylpyrrolidone polymer use in both wet and dry electrophotographic processes. selected from the group consisting of poly-N-vinyl pyr- Resistivity (ohn1s 10 /sq. inch) Holdout (111111.) 20% RH 50% R11 80% R11 Solvent Tonel Surface Surface Silllutv Forniu- Paper lolyliydric 7 Ex. lation surface alcohol Felt Wire Fell; Wire Felt Wire Bulk Felt Win- Bulk l-vlt Wire Bulk 5... Ii D1820... 1,4-butanediol..... 87 75 72 70 15 15 .2 0.28 0.21 0.011 0. 0010 .0010 00081! 6... I .do.. Ethylene glycol... 85 77 7 74 20 2.7 0.20 0. 23 0. 051 0. 0010 .0010 00080 7... J ..do. GafanolE200...... 20 20 1.5 0.12 0. 004 0. 00 0. 0014 .0013 00080 8.. K .do. GafanolE300 1 10 10 1.7 0.22 0.15 0.071 0.0013 .0012 00030 A .110... Glycerol 65 70 66 71 0.1 0.0..) 0.05

Examples 10-15 rolidone, N-vinylpyrrolidone/vinyl acetate copolymer, N-vinyl rrolidone eth lacr late co ol mer and mixtures thereof, These examples illustrate the suitability of the inorganic 2O py y p y I I it 1 h d l h l b d h d b 2.5 to 5 percent by weight ofa polyvinyl acetate emulsion, 2 to 5a p0 i x a co 3 mgs W en app y 5126 6.5 percent by weight of an acetylated starch and 2 to 6.5 perprgss w i? ca en d d b cent by weight ofa polyvinyl alcohol, said inorganic salts hav' gatmg cqmpoSl Ions prepfare e 0V6 were ing a nonvolatile, conductive anion, and having a cation p r h to vii-1051s paper sur aces S lrotug a E grass 25 selected from the group consisting of the ions of ammonium, mac s g iig O u er z lithium, sodium, potassium, rubidium, and cesium, and one g E 2 5) .were nee e O reac face of said base having a coating thereon overcoated with a eslre P or CPmPaHSOHPurPOSeSKa commerc.la second coating containing a photoconductive zinc oxide pigpaper containing an organic conductive agent in the barrier mem film and paper coated by air knife techniques with a composition ofthis invention were also evaluated. 2. Electrophotographic recording material as defined in Solvent and toner holdout were determined by the claim 1 wherein the inorganic salt is sodium chloride and the procedures described earlier. The size-press-applied coatings polyhydric alcohol is glycerol. offered excellent resistance to both solvent and toner absorp- I I d f d tion. Surface and bulk resistivities, determined at several hu- Electrophotogrflphlc i 8 me 5 claim 1 wherein the inorganic salt is sodium chloride and the midities using the procedure described earlier, show that the inorganic salt-polyhydric alcohol-coated papers were acceptable for electrophotographic use in either wet or dry development processes.

Details as to the compositions of the coatings, the surfaces coated, the application techniques and the data obtained are tabulated below.

polyhydric alcohol is ethylene glycol.

4. Electrophotographic recording material as defined in claim 1 wherein the inorganic salt is ammonium chloride and the polyhydric alcohol is glycerol.

5. Electrophotographic recording material as defined in Resistivity (ol1nisX10/sq. inch) lloldout (111111.) Surface Bulk Forniu- Paper Dis- Sainplu lation surface Method of application Solvent porsant 20% R11 R11 80% R11 20% lill 50% R11 80% R11 10... E. DiazO. Size press and 08 G4 .22 0. 28 0.0013 2.41 0. 054 0. 00058 calender. 11. E. Pattern. .(10 S0 73 24 0. 27 0, 0010 l. .2 U. 058 0. 00052 12... E. 4107-0, MF .....(10 70 G0 50 0.51 0.0068 .2. T 0, 77 (I. 0010 13.. C. 4107-0, MF. Air knife and 04 8'. 3t: 0. 34 0. 0010 1.5 0. 085 ll. 00068 calender. 14.. Z0. ..(10 84 .25 0. 20 0. 0015 1. 0 0. 085 0.0005 15.. Con Trailing blade J7 83 4. 4 0. 10 0. 0040 (1. 72:1 0. 015 0. 0023 While exemplary embodiments of the invention have been claim 1 wherein the N-vinylpyrrolidone polymer is a copolymer of N-vinylpyrrolidone and vinyl acetate in which the proportion of N-vinylpyrrolidone in the copolymer is from 10 to 25 percent by weight. 

2. Electrophotographic recording material as defined in claim 1 wherein the inorganic salt is sodium chloride and the polyhydric alcohol is glycerol.
 3. Electrophotographic recording material as defined in claim 1 wherein the inorganic salt is sodium chloride and the polyhydric alcohol is ethylene glycol.
 4. Electrophotographic recording material as defined in claim 1 wherein the inorganic salt is ammonium chloride and the polyhydric alcohol is glycerol.
 5. Electrophotographic recording material as defined in claim 1 wherein the N-vinylpyrrolidone polymer is a copolymer of N-vinylpyrrolidone and vinyl acetate in which the proportion of N-vinylpyrrolidone in the copolymer is from 10 to 25 percent by weight. 